1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to a method of controlling a handover by combining both a soft handover and a hard handover in an uplink synchronous transmission scheme (USTS).
2. Background of the Related Art
Generally, an uplink synchronous transmission scheme (USTS) is used to reduce multiple access interferences using orthogonality, by controlling reception timing between mobile stations and a base station in a closed loop timing control manner.
A related soft handover manner uses a technique that a mobile station (e.g., a user equipment or a mobile terminal) communicates with a plurality of base stations and continuously maintains a communication channel during the time of the handover. Here, the soft handover manner can be applied to a softer handover between sectors.
The handover manner can generally be divided into: (1) a hard handover that terminates a communication channel of a present source base station before connecting the communication channel to a target base station; (2) a soft handover that terminates the communication channel of the source base station after connecting the communication channel to the target base station; and (3) a softer handover that performs the soft handover function between sectors in the same base station, which is divided into multiple sectors. The soft handover maintains the communication channel and concurrently connects the same communication channel to two base stations serving the handover during a constant time. The hard handover causes discontinuity for the communication channel; a new communication channel is connected after terminating the present communication channel.
FIG. 1 is a schematic block diagram of a related mobile communication system for illustrating a handover function between two base stations in an uplink channel. Referring to FIG. 1, the related mobile communication system includes a radio network controller 101 for selecting an optimized frame on the basis of a received radio frame, a mobile station 102, and more than one base station 103 or 104. The radio network controller 101 includes a selector 101a for selecting the optimized radio frame.
A transmission signal of the mobile station 102 is concurrently received at the source base station 103 and the destination base station 104 in the uplink channel. Handover is performed in the related mobile communication system when the provisioning of service for the mobile station 102 is moved from a cell area of the source base station 103 to a cell area of the destination base station 104.
The source base station 103 and the destination base station 104 demodulate the transmission signal received from the mobile station 102 and then forward it to the radio network controller 101 with the radio frame. Then, the radio network controller 101 can select the optimized transmission signal on the basis of each respective transmission signal. Accordingly, the radio network controller 101 can be connected to the mobile station 102 via the communication channel of the base station corresponding to the optimized transmission signal.
Meanwhile, the softer handover serves the same operation as the soft handover function described above, if the mobile station 102 is moved from one sector to another sector in the service area of the base station. That is, in case of the softer handover, the two signals received from the mobile station 102 are demodulated in the source base station 103 or the destination base station 104 and then one of the two demodulated transmission signals is forwarded to the base station subsystem 101.
FIG. 2 is a block diagram of a related mobile communication system illustrating the soft handover function between two base stations controlled by the same radio network controller. Referring to FIG. 2, the related mobile communication system includes a core network 201, a mobile station 220, and a UMTS radio connection network 210 connected between the core network 201 and the mobile station 220. The UMTS radio access network 210 is a generic term for the related radio network controller(s) and base station(s). The UMTS radio access network in FIG. 2 includes a serving radio network controller (SRNC) 211 and at least a base station 212 or 213 connected with the SRNC 211. The SRNC 211 manages the dedicated radio resources allocated to the mobile station 220. If the mobile station 220 is moved from the service area of a source base station 212, which refers to the base station that is providing service to the mobile station, in an area of the SRNC 211 to the service area of a target base station 213, the source base station 212 and the target base station 213 demodulate the transmission signal received from the mobile station 220 and then transmit the demodulated signal to the SRNC 211 with the radio frame form, respectively. The SRNC 211 selects the optimized transmission signal on the basis of the respectively received transmission signal and thereby continuously maintains the communication by concurrently connecting the communication channel for the mobile station 220 to the two base stations 212 and 213 in the area where the cell service areas of the source base station 212 and the target base station 213 overlap.
FIG. 3 is a block diagram of the related mobile communication system serving the soft handover between two base stations when radio network controllers controlling at least two more base stations are different from each other. Referring to FIG. 3, although the mobile communication system is very similar to the system shown in FIG. 2, a drift radio network controller (DRNC) 312 has been added in the UMTS radio access network 310. Also, the mobile communication system includes at least two radio network controllers 312 and 316 which control the transmission signal received from the mobile station 220. These two radio network controllers 312, 316 are different from each other. The SRNC 316 and the DRNC 312 are linked to base stations 313 or 317 that can be controlled, respectively. FIG. 3 shows the related mobile communication system serving the soft handover function from the cell area of the source base station 317, controlled by the SRNC 316, to the cell area of the target base station 313 controlled by the DRNC 312. Accordingly, if the mobile station 220 moves away from the cell area of the source base station 317 toward the cell area of the target base station 313, the mobile station 220 can concurrently maintain the communication channel for each base station 313 and 317 as a result of a communication channel between the SRNC 316 and the DRNC 312. Initially, the SRNC 316 manages the dedicated radio resources allocated to the mobile station 220 and the DRNC 312 can provide the radio resources to the mobile station 220 when the mobile station 220 is moved from the cell area of the first base station 317 to the cell area of the second base station 313.
FIG. 4 is a flow chart illustrating the handover control procedure for the related soft handover. Referring to FIG. 4, the method of controlling the handover can be applied to the case of controlling the handover when the mobile station 102 is moved from the source base station 103 to the target base station 104, as shown in FIG. 1. Also, the method can be applied to the case in which the radio network controller(s) controlling each base station is/are the same or different each other.
Initially, the communication channel of the mobile station is connected only through the source base station, when the mobile station is within the service area of the source base station. But, if the mobile station enters the area overlapped by the service cell areas of the source base station and the target base station, thereby initiating the handover (step 401), the mobile station connects the communication channel not only to the source base station but also to the target base station.
As described above, the mobile station periodically measures the strength of a pilot signal received from at least one base station and transmits the measured value to the radio network controller via the base station. Then, the radio network controller selects an optimized pilot strength on the basis of the measured values. The radio network controller can connect to the communication channel with the base station in which the optimized pilot strength has been measured by the mobile station. Accordingly, the mobile station measures the pilot strengths of the source base station and the target base station (step 403) and determines whether the pilot strength measured in the target base station exceeds a predetermined pilot reference value. If the measured pilot strength exceeds the pilot reference value, then the mobile station transmits the measured results of the source base station and the target base station to the radio network controller. Additionally, the target base station is identified in a candidate list for serving the handover (step 405).
The radio network controller determines whether to control the handover based on the measurements of the pilot strengths of the source and the target base stations. If it is determined that the pilot strength of the target base station is sufficient, then the radio network controller transmits the first handover message to the mobile station to set up the communication channel with the target base station. That is, if the radio network controller transmits the first handover message to the mobile station, the mobile station starts communicating with the target base station on the basis of the first handover message via a new communication channel. Here, the first handover message includes a PN offset of the target base station and newly allocated Walsh codes. Accordingly, the mobile station transfers the target base station from the candidate list onto an actual communication list and transmits a handover completion message to the radio network controller, after obtaining a synchronization of a downlink communication channel defined in the first handover message. Thus, the mobile station communicates with both the source base station and the target base station (steps 407 and 409).
According to the description above, the communication channel is established between the mobile station and the target base station by the first handover message of the radio network controller, when the mobile station enters to the cell area of the target base station. Meanwhile, the mobile station periodically measures the pilot strengths of the source base station and the target base station (step 411 step) and determines whether the pilot strength of the source base station is enough or not (step 413). That is, the mobile station determines whether the pilot strength of the source base station is less than a pilot extracting reference value. The mobile station starts measuring the pilot strength of the source base station to determine if it is less than the pilot extracting reference value. If the pilot strength reaches the predetermined extracting threshold value, the mobile station transmits the pilot strength-measuring message to the radio network controller. So, the mobile station receives the second handover message from the radio network controller (step 415).
The mobile station deletes the source base station from the actual communication list on the basis of the received second handover message and transmits the handover completion message to the radio network controller (step 417). Here, the second handover message only includes a PN offset for the target base station and does not include the PN offset for the source base station. As described above, the method of releasing the communication channel of the source base station is explained in the case where the pilot strength of the source base station is less than the pilot extracting reference value.
FIG. 5 is a time sequential chart illustrating the data flow procedure for protocol entities of each communication element, when the related soft handover is performed. FIG. 5 illustrates the data flow procedure in the case where different radio network controllers, controlling at least one base station, are in operation. Also, the same procedure can be applied to the case where more than one base station is controlled by the same radio network controller. That is, the description for the data flow procedure of FIG. 5 can be clearly understood with reference to FIG. 3.
Referring to FIG. 5, the data flow procedure can be classified into a radio link adding procedure (step 420) and a radio link deleting procedure (step 433). Each procedure will be explained in detail below. Here, step 420 explains establishing the communication channel through the radio link between the target base station 327 and the mobile station 329 when the handover is performed, while step 433 explains releasing the radio link established between the source base station (SRNS-base station) 325 and the mobile station 329.
In step 420, if the mobile station 329 enters into a handover area, the serving radio network controller (SRNC) 321 decides whether to set up a new link between the mobile station 329 and a target base station (DRNS-base station) 327 (step 419). Here, the handover area means an area overlapped by cell areas of the source base station 325 and the target base station 327, as described above. Of course, the SRNC 321 has to be operated to receive the resulting pilot strengths measurements from the source base station 325 and the target base station 327 before performing step 419. Accordingly, the SRNC 321 can decide to set up the new link on the basis of the measured results.
If it is necessary to establish the new link between the mobile station 329 and the DRNS-base station 327 as a result of the decision, the SRNC 321 transmits a radio link setup request message to the DRNC 323 by using a Radio Network Subsystem Application Part (RNSAP), which is an interfacing protocol between the radio network controllers (step 423). The radio link setup request message includes a command for establishing the new radio link. The DRNC 323 transmits the radio link setup request message to the DRNS-base station 327 using a Node B Application Part (NBAP), which is a protocol between the base station and the radio network controller (step 423). “Node B” represents a base station.
The DRNS-base station 327 transmits a radio link setup response message to the DRNC 323, using the NBAP protocol, after successfully establishing the radio link with the mobile station 329 and on the basis of the radio link setup request message (step 425). Here, the radio link setup response message includes a report of successfully establishing the radio link between the mobile station 329 and the DRNS-base station 327. The DRNC 323 transmits the radio link setup response message to the SRNC 321 by using RNSAP protocol (427 step).
Subsequently, the SRNC 321 transmits an active set update command message to the mobile station 329 using the radio resource control (RRC), which is an interface protocol used between the mobile station and the radio network controller. Then, the mobile station 329 can add the DRNS-base station 327 to an active set on the basis of the active set update command message (step 429). Here, the active set means a group of base stations that are communicating with the mobile station through the same downlink communication channel. The mobile station 329, after adding the DRNS-base station 327 to the active set, transmits an active set update complete message to the SRNC 321 by using the RRC protocol (step 431).
Next, in step 433, if the mobile station 329 moves away from the handover area and enters a cell area of the DRNS-base station 327, the SRNC 321 decides whether to remove a predetermined radio link (step 435). Here, the predetermined radio link means a communication channel between the SRNS-base station 325 and the mobile station 329. Of course, the measured value of the pilot signal, for the SRNS-base station 325, that is measured at the mobile station 329 has to be transmitted to SRNC 321 before performing step 433. Accordingly, the SRNC 321 can decide whether to remove the radio link established at the SRNS-base station 325 based on the measured signal value of the SRNS-base station 325.
As a result, in the case of removing the radio link established on the SRNS-base station 325, the SRNC 321 transmits the active set update command message to the mobile station 329 using the RRC protocol (step 437). The mobile station 329 removes the radio link established on the present SRNS-base station from the active set on the basis of the active set update command message received from the SRNC 321 and transmits an active set update complete message to the SRNC 321 using the RRC protocol (step 439).
The SRNC 321 transmits a radio link deletion request message to the currently serving SRNS-base station 325 using the NBAP protocol (step 441). Then, the serving SRNS-base station 325 releases the radio link between the serving SRNS-base station 325 and the mobile station 329, according to the radio link deletion request message, and transmits a radio link deletion response message to the SRNC 321 using the NBAP protocol (step 443).
It is noted that the remaining steps, after excluding steps 421 to 427, may be the same used if more than one base station is controlled by the same radio network controller. That is, the command of the SRNC is directly transmitted to the SRNS-base station 325 without passing through the DRNS-base station as shown in steps 421 to 427, since the DRNS-base station 327 does not exist in the case where more than one base station is controlled by the same radio network controller.
As described above, the related art method for controlling the handover does not consider a transmission scheme that is provided to improve performances of the uplink in the base station, i.e., an uplink synchronous transmission scheme for synchronizing the reception timing between the mobile stations. That is, the related handover is served without considering the transmission scheme for controlling the reception timing of the mobile stations by using the closed loop timing control scheme in the base station.
Accordingly, in order to improve the method of controlling the handover, a hard handover means of the uplink synchronous timing essentially required in the uplink synchronous transmission scheme should be added together with the related art soft handover method. In the present invention, it should be noted that the hard handover of the uplink synchronous timing is used instead of the soft handover, since only one of the base stations is selected, because synchronizing the uplink synchronous timing for the source base station and the target base station cannot occur at the same time.
If the hard handover of the uplink synchronous timing is not considered, there are problems of not utilizing the uplink synchronous transmission scheme function and decreasing the reception capacity and cell coverage areas due to a failure in maintaining the uplink synchronous transmission gain by moving the mobile station between the sectors or base stations.
In the long run, it is expected that a new method of controlling the handover, wherein the hard handover manner controlling the synchronous timing is used in conjunction added with the related soft handover manner, will supplement the related art method in the field.